The term "immunomodulation" denotes an intended physical or chemical alteration in the function of a host's immune system. The distinction is made between manipulations which cause a decrease and others which cause an increase in number and/or function of immune mechanisms. The desire and necessity to artificially control immune responses were born during the advent of clinical transplantations in the years 1965-75 when the immunologists were faced with the daunting task of preventing graft rejections. Hence was born the era of "Biological Response Modifiers (BRM's)" which can be classed as either immunosuppressive or immunostimulatory. A special subgroup of the latter exerts its stimulatory effects only on defective immune mechanisms and directs them towards normalization. Such substances are termed "immune restorative agents".
In the past, living and attenuated micro-organisms, autologous and heterologous proteins and injections of animal organ preparations were used with the aim of restoring an impaired defence mechanism. At present, thymic peptides, synthetic low molecular weight compounds, chemically modified nucleotides. polysaccharides from fungi and especially, some plant extracts are also used for the same purpose.
The recently reviewed interest in the immunomodulators arose from their potential therapeutic use in several clinical situations including:
a) chronic bacterial and/or viral infections PA0 b) immune dysfunction PA0 c) immune deficiency syndromes PA0 d) tumours
Although an extensive search for plant derived natural products with immunostimulatory/immunosuppressive activities is currently under way, very few such substances have entered the market.
Biological activities of some steryl glycosides have been investigated such as their effects on complement activation and their anti-inflammatory activity. The steryl glycosides, also known as phytosterolins, include the glycosides of .beta.-sitosterol, stigmasterol, campesterol and the like, but the majority of the work has been carried out with .beta.-sitosterol glucoside and its aglycone. .beta.-sitosterol glucoside is obtained from a variety of plant material extracts in the form of a mixture containing, in most cases, not less than 60% of .beta.-sitosterol glucoside with other steryl glycosides and in this specification the term .beta.-sitosterol glucoside (which is abbreviated to BSSG) can mean either the substance itself or a mixture thereof with the other steryl glycosides. The same applies to the aglycone .beta.-sitosterol (abbreviated to BSS in this specification).